GNAS and pseudohypoparathyroidism type 1A: Pseudohypoparathyroidism (PHP) and related disorders are associated with a spectrum of abnormal physical characteristics as well as neurocognitive and endocrine abnormalities that are caused primarily by molecular defects that impair hormonal signalling via receptors that are coupled, through the α-subunit of the stimulatory G protein (Gsα), to activation of adenylyl cyclase (Fig. 1).